The tenten marks voicing on the consonant part of the が mora (syllable). Although there are cases where vowels are devoiced, there's no mark for it in ordinary writing.

Anyway, you can certainly have a long vowel that starts with a voiced more.があ isn't really a word ... although there is ガー ... which Edict assures me means 'Gar', a type of fish. The katakana dash is also a long vowel, and although I think you won't find a native Japanese word with があ in it, if you did it would be indicating a long あ vowel.

Yes, I've just realized my mistake. I meant ぴい, this is possible (theoretically, I don't know if there is such a word)But when we write Tokyo in Hiragana it is とうきょう and the last part is a combo with long vowel, isn't it? But the first part と and う are two separate sounds?

The ー character is not part of katakana. In the post-WWII writing standards, it's used to indicate long vowels in 外来語 written in katakana, but not in other words in katakana such as the scientific names of plants, animals, etc. It's also often used with hiragana when writing some long vowels in dialect words and onomatopoeia. Pre-war it was used more widely with hiragana.